Ecto-Nicotinamide Adenine Dinucleotide Oxidase Disulfide-Thiol Exchanger (ENOX) proteins are a family of cell surface enzymes that exhibit both a hydroquinone (NADPH) oxidase and a protein disulfide-thiol interchange activity (Morré and Morré, 2013, Ecto-Nox Proteins. New York: Springer). These two enzymatic activities are essential for the expansion phase of cell proliferation (Morré and Morré, 2003. Free Radical Res. 37: 795-808). Unlike other human ENOX family members that are regulated by growth factors, the enzymatic activity of ENOX2 (also called ECTO-NOX2 or tNOX) is constitutively active and unresponsive to hormone signaling. Importantly, ENOX2 is completely absent from normal cells, except during the early stages of embryogenesis where ENOX2 activity may contribute to rapid cell proliferation. However, ENOX2 expression is also invariably induced during the benign to malignant transition of cancer cells. Thus, ENOX2 represents an oncofetal antigen and its expression universally coincides with oncogenic progression leading to malignancy.
ENOX2 is a terminal hydroquinone oxidase of the plasma membrane electron transport chain that is universally produced by malignancies, and the activity of ENOX2 significantly contributes to the unregulated growth that typifies cancer. Inhibitors of ENOX2 induce apoptosis in cancer cells, but not in non-cancer cells, an indication that the enzymatic activity of ENOX2 strongly contributes to the unregulated expansion and survival of cancer cells.
Tissue-specific ENOX2 isoforms are produced as a result of differential mRNA splicing (Tang et. al., 2007. Oncol. Res. 16: 557-567; Tang et. al., 2007. Biochemistry 46: 12337-12346). Therefore, different ENOX2 isoforms are expressed by cancers of different cellular or tissue origin. However, each ENOX2 isoform contains an identical core domain that is essential for ENOX2 enzymatic activity, allowing for immunodetection of the core region of all ENOX2 isoforms by using a single recombinant antibody (Hostetler et. al., 2009. Clin. Proteomics 5: 46-51).
ENOX2 protein expression is detected exclusively in the sera of cancer patients, but is completely absent from the sera of non-cancer patients. Therefore, unlike most cancer markers, ENOX2 is not simply produced at higher levels by cancer cells relative to non-cancer cells. ENOX2 expression has been detected from every type of malignancy tested to date, including: bladder, breast, cervical, colorectal, endometrial, esophageal, gastric, liver, kidney, lung (small and non-small cell), ovarian, pancreatic, prostate, follicular thyroid, papillary thyroid, and endometrial (uterine) cancers (Morré and Morré, 2013, Ecto-Nox Proteins. New York: Springer; Hostetler et. al., 2009. Clin. Proteomics 5: 46-51). The wide diversity of cancer types that have been found to produce ENOX2 indicates that ENOX2 expression is a conserved feature of most, and potentially all, malignancies. Thus, the cell-surface ENOX2 enzyme emerges as a universal cancer marker.